Prior art thermal transient testers employ a manually balanced wheatstone bridge. This bridge comprises two fixed resistors in two branches; an electro-explosive device under test forming a third branch and a set of variable resistors forming a fourth branch. The electro-explosive device comprises a pyrotechnic device having a pair of terminals to which is connected a bridge wire of relatively low resistance. A pyrotechnic material encases the bridge wire. When an electric current pulse of sufficient amplitude is applied to the device terminals, the bridge wire is heated to a point where the ignition temperature of the pyrotechnic material is reached and the pyrotechnic material ignites. It is important to test these pyrotechnic devices for proper resistance values of the bridge wire and also for the thermal response characteristics of the bridge wire to ensure that the pyrotechnic material will in fact ignite when an ignition current pulse is applied to the device terminals. Otherwise failure of the device to ignite creates safety and other problems.
For these reasons the transient tester mentioned above is utilized in the prior art for determining both the resistance of the device and for testing the thermal response characteristics of the device to ensure the device meets its specified requirements so that it will in fact ignite in response to an appropriate ignition current signal applied thereto.
To test such devices for thermal transient response, the device is inserted as one arm or branch of the manually balanced bridge. The variable resistors are shifted in value for nulling the bridge. A current pulse is then applied to the bridge input nodes. The current pulse has an amplitude sufficient to heat the bridge wire of the electro-explosive device an amount sufficient to generate a thermal response in the bridge wire. The thermal response is then measured and observed on an oscilloscope for appropriate analysis. This approach is cumbersome and awkward and, further, the number of devices tested is limited to several per minute in response to the need for an operator to balance manually the bridge for each device, test the device, remove the device and insert a new device and rebalance the bridge for the new device.
The bridge is balanced or nulled in order to cancel the effect of the resistance value of the electro-explosive device under test. In this way, the thermal response characteristic curve of the device is a measure only of the thermal response of the device and does include the actual resistance of the device which may complicate the test result.
The present invention is directed to a need recognized by the present inventors for an automatic system for testing the electro-explosive device described above automatically and rapidly. This automatic testing of the device results in several problems including automatically measuring the device resistance and automatically balancing the bridge so as to achieve a null condition for applying a thermal transient current signal to the device.